Test-driven development (TDD), also called test-driven design, is a method of software development in which unit testing is repeatedly done on source code. The concept is to "get something working now and perfect it later." After each test, refactoring is done and then the same or a similar test is performed again. The process is iterated as many times as necessary until each unit is functioning according to the desired specifications. Test-driven development is part of a larger software design paradigm known as Extreme Programming (XP).